Heat recovery steam generators (HRSG) are used to recover heat contained in the exhaust gas stream of a gas turbine or similar source and convert water to steam. In order to optimize the overall plant efficiency, the HRSG includes one or more steam generating circuits which operate at selected pressures.
Natural and assisted circulation boilers utilize a steam drum operating at high pressure in which the steam is separated from the water. The steam drum should be of sufficient volume to allow for changes in water level in the drum due to changes in fluid density and operating conditions. Since the length of the drum is often limited by space constraints, the needed volume is obtained by increasing the inside diameter of the drum. However, to maintain acceptable stress levels on the drum walls, the thickness of the drum wall must be increased when the inner diameter is increased.
Increased drum wall thickness increases the through wall temperature gradient at boiler start-up since it takes a longer amount of time to heat through a thicker wall. This increase in through wall temperature gradient results in increased thermal stress on the steam drum wall, which may, in turn, cause wear of the steam drum wall in the form of cracking. Cracking of the steam drum wall necessitates maintenance or repair which causes plant shut-down and added expense.
The configuration and methods of operation described herein are seen to reduce or eliminate the above mentioned deficiencies that may be found in current systems.